Chemistry and Analysis of Hop and Beer Bitter Acids

423 Pages·1991·19.898 MB·English

by  M. Verzele, D. De Keukeleire

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CHAPTER 3
REDUCED DERIVATIVES OF HUMULONE
Reduction, achieved in various ways, is an established means of chemical
transformation and structural investigation.
3.1. DIHYDROHUMULONE.
When humulone (or another alpha acid) is hydrogenated in acidic conditions,
especially with palladium as catalyst, an alkenyl side chain is removed. To avoid this
(rare example of) hydrogenolysis (see 3.3.) humulone is hydrogenated with
platinum(IV) oxide in methanol at pH 5.1. 2-(3-Methylbutanoyl)-6-(3-methyl-2-butenyl)-
4-(3-methylbutyl)-3,5,6-trihydroxycyclohexa-2,4-dienone or dihydrohumulone (18, Fig.
15) is isolated by counter-current distribution in the two-phase system benzene :
aqueous buffer pH 8.4 (15% triethanolamine, 25% ethylene glycol and 0.25 N HCI) (1).
After 330 transfers 18 is found as a band with distribution coefficient of 1. It occurs as a
light-yellow oil of elemental composition C21H32O6.
The spectrometric characteristics of 18 are very similar to those of humulone.
However, in the 1H NMR spectrum the conversion of the 3-methyl-2-butenyl side chain
into the 3-methylbutyl group can clearly be discerned (2). Around 5 5a triplet for only
one sp2-methine proton is found, while two methyl groups on a double bond remain.
Next to a doublet at 8 0.9, attributed to the two geminal methyl groups, the remaining
hydrogen atoms of the 3-methylbutyl side chain display a complex absorption pattern
between 8 1.2 and 8 2.2.
The structure can also be proved by the 13C NMR spectrum (3). Compared to
humulone, all chemical shifts agree up to 0.2 8, except for the signals of the carbon
atoms in the 3-methylbutyl side chain. The ring carbon atom, to which the chain is
attached, is found at 8 110.7. The carbon atoms of the methylene groups occur at 8
19.8 and 8 37.6, respectively. The methine carbon atom absorbs at 8 28.2 and the
methyl groups display signals at 8 22.6 and 8 22.8, respectively. It is remarkable that
only one of the two isomers of dihydrohumulone is formed. Alkaline degradation of the
experimentally obtained isomer produces only dihydrohumulinic acid (19, Fig. 15)
while the corresponding humulinic acid (20, Fig. 15) is not detected, at least not in
larger quantities. It would be interesting to check this chemistry with HPLC techniques.
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At the time of these investigations that possibility did not yet exist. Similar selectivity is
often encountered in hydrogenation reactions (4).
HO OH
20
Fig. 15. Structural formulae of dihydrohumulone (18), dihydrohumulinic acid (19) and
humulinic acid (20).
3.2. TETRAHYDROHUMULONE.
The hydrogenation, performed in appropriate conditions, as described in section
3.1., gives tetrahydrohumulone (83%) or 2-(3-methylbutanoyl)-4,6-bis(3-methylbutyl)-
3,5,6- trihydroxycyclohexa-2,4-dienone (5, Fig. 2) after uptake of 2.35 moles hydrogen
(4-6). Tetrahydrohumulone has a K value of 2 in the same two-phase system as
applied for the separation of compound 18 (see 3.1). After purification by normal phase
chromatography on silica gel with benzene as eluent, compound 5 is isolated as a
light-yellow crystalline material (molecular formula : C21H34O6) with melting point
58.5-59.5°C (7). The specific optical rotation at the Nap-line is -116 in methanol and
the complex with 1,2-phenylenediamine has a melting point of 96°C.
In the 1H NMR spectrum no signals appear for methyl groups or protons on
sp2-carbon atoms. A number of doublets, observed between 5 0.8 and 5 1.0, can be
attributed to six geminal methyl groups. The doublet of the methylene protons in the
3-methylbutanoyl side chain is found at 8 2.8, while all other hydrogen atoms, bonded
to carbon, display a poorly resolved pattern in the region 8 1-3 (2).
Racemic tetrahydrohumulone (melting point 84°C) was already obtained in 1925,
by hydrogenolysis of the beta acid lupulone and subsequent air oxidation of the
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intermediate 4-deoxytetrahydrohumulone (see 12.3.) (8). Tetrahydrohumulone,
obtained by hydrogenation of (-) humulone, is optically active of course. It is not
possible to racemize such optically active tetrahydrohumulone, while this can be done
for (-) humulone. The racemate can also be obtained in 75% yield by racemization of
(-) humulone under pressure at 125-130°C, followed by hydrogenation (7). This
racemization therefore requires the presence of the double bonds and proceeds via a
crypto-ionic mechanism, in which the double bond in the side chain at the chiral centre
plays a central role (Fig. 16).
Hydrogenation of humulone.
Humulone (10 g; 2.76 x 10'2 mol) is dissolved in absolute methanol (200 ml). The
pH of the solution is adjusted to 5.1 with methanolic potassium hydroxide (2 N). The
catalyst (5% platinum(IV) oxide) is added as a suspension in methanol. The
hydrogenation is stopped after absorption of 2.35 mo Is hydrogen. After filtration and
removal of the solvent, the residue is separated by counter-current distribution in the
two-phase system benzene : triethanolamine (15%), ethylene glycol (25%) and HCI
0.25 N (pH 8.4). Dihydrohumulone has a K-value of 1 after 330 transfers and
tetrahydrohumulone is distributed with a K-value of 2. Residual humulone is found in
the band with K0.4.
3.3. HUMULOHYDROQUINONE - HUMULOQUINONE.
The hydrogenolysis of humulone, leading to humulohydroquinone or
4-(3-methylbutanoyl)-6-(3-methylbutyl)-1,2,3 5-tetrahydroxybenzene (21, Fig. 17), is a
>
side reaction in the hydrogenation (8-10). The corresponding cleavage of a
3-methyl-2-butenyl group in the hop beta acids (see 12.2.) proceeds even more
efficiently. On the contrary, no hydrogenolysis is observed when the side chains are
saturated. The reaction occurs only with a so-called active side chain, e.g. allyl, crotyl,
isoprenyl or benzyl; the reactivity follows this order (11).
The driving force for the hydrogenolysis is the aromatization in cooperation with
the particular nature of the side chain stabilizing the intermediate (crypto) allylic or
benzylic cation. The reaction starts with a reversible proton addition to the carbonyl
function and proceeds via addition of a hydride ion to an allylic carbon atom.
Concurrently or subsequently the double bond in the side chain is hydrogenated (Fig.
12).
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/ ii *»
Fig. 16. Mechanism for the racemization of (-) humulone.
To confirm this reasoning, hydrogenations of humulone have been carried out with
platinum, palladium and rhodium catalysts at several pH values. A linear relationship
between the pH and the logarithm of the percentage hydrogenolysis has been
determined experimentally. At a given pH value, palladium catalysts are more active
than platinum or rhodium catalysts. This is in accordance with the theoretical
requirements for the intervention of a hydride ion (12).
Humulohydroquinone is oxidized by air to the red-coloured humuloquinone or
2-(3-methylbutanoyl)-6-(3-methylbutyl)-3,5-dihydroxy-p.-quinone (22, Fig. 17). Both
derivatives have been synthesized and found to be identical with the natural
compounds (13).
, OH 0 , I O Ol
OH °
2L 21
Fig. 17. Formation of humulohydroquinone (21) and humuloquinone (22).
Preparation of humulohydroquinone - humuloquinone.
A solution of humulone (0.558 g; 1.54 x 10~3 mol) in methanol (15 ml) with 10%
palladium(ll) chloride in concentrated HCI (0.6 ml) is shaken with hydrogen. After
absorption of 2.76 mol hydrogen (1.5 h) the reaction rate decreases. After filtration,
dilution with water and extraction with ether, the humulohydroquinone solution is
oxidized with air during several hours. The ether layer is shaken with sodium
hydroxide 0.5 N during 3 min to complete the oxidation. The acidified solution is
extracted again with ether. Removal of the solvent and recrystallization from methanol:
water yield humuloquinone (0.405 g; 90%). Several recrystallizations produce a
deep-red crystalline compound with melting point 73-73.5°C (Xmax : 300 nm and 440
nm in CH OH : HCI 0.1 N; 310 nm, 350 nm (shoulder) and 530 nm in MeOH : NaOH
3
0.1 N).
3.4. 4-DEOXYHUMULONE.
As described under 2.3. and 2.4. compound 12 (Fig. 18) is an intermediate, as well
in the biogenesis as in the laboratory synthesis of humulone. It has been isolated from
hops and has been synthesized by different methods (see 2.4.1. and 2.4.2.).
Remarkably, 4-deoxyhumulone or 2-(3-methylbutanoyl)-4,6-bis(3-methyl-2-bute-
nyl)phloroglucinol is directly accessible from the hop beta acid lupulone (23, fig. 18).
Upon irradiation a 3-methyl-2-butenyl side chain is photolysed. This process has been
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presented as occurring via a six-membered ring mechanism with migration of the
proton on the double bond of the 3-methyl-2-butenyl side chain (14,15).
Since hexahydrolupulone is unreactive in similar conditions, it is more likely that
the formation of 12 involves a Norrish Type II cleavage (14). In this event the (3-bond
with respect to the carbonyl group of the ring is cleaved, thereby giving rise to two
stabilized radicals (16) (Fig. 18). No photolysis is observed for hexahydrolupulone,
since the formation of an allyl radical is not possible. Because 4-deoxyhumulone can
be oxidized to humulone, this reaction sequence represents a method for converting
the hop beta acids to the hop alpha acids. 4-Deoxyhumulone is a reduction product of
humulone and can be obtained from humulone by electrolysis (see 2.4.2.).
Considering the great sensitivity to oxidation of deoxyhumulone it would be
expected that hop extracts, especially after some time, would not contain deoxy alpha
acids. They do however, and the detectable amounts are even remarkably constant.
This fact suggests that either the alpha acids or the beta acids in hops can be
converted into deoxy-alpha acids. This possibility would seem to warrant further
research.
Fig. 18. Mechanism for the formation of 4-deoxyhumulone.
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Conversion of lupulone to 4-deoxvhumulone.
Lupulone (2.08 g; 5 x 10~4 mol) is irradiated at 350 nm in methanol (500 ml) under
nitrogen during 7 days. The reaction can be monitored by the disappearance of the UV
absorption band at 255 nm and the appearance of a new UV maximum at 290 nm.
After removal of the solvent, the residue is recrystallized from pentane (65-75%).
UV: Xmax (e) : 290 (18280) nm and 235 (3280) nm in ethanol.
IR: vmax (CCI4) : 3600, 3400, 1665, 1615, 1595, 1300-1450, 1380, 1230 cm^'.
1H NMR : (60 MHz; CCI ; TMS) : 8:1.1 (6H, d); 1.7 (1H, m); 1.9 (12H, d); 2.9 (2H, d);
4
3.3 (4H, d); 5.2 (2H, t).
Mass spectrum of the tris(trimethylsilyl) ether (El) : m/z (%) : 562 (2); 547 (24); 519 (8);
507 (24); 491 (4).
3.5. 2,6-BIS(3-METHYLBUTYL)-4-(3-METHYLBUTANOYL)RESORCINOL.
This aromatic compound (6, Fig. 19) is formed by perhydrogenation of
tetrahydrohumulone in acidic medium (see 2.1.1.2.1.c).
Fig. 19. Perhydrogenation of tetrahydrohumulone.
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After reduction of the carbonyl group to a methylene function, compound 6 (Fig.19)
is obtained by dehydration involving the tertiary alcohol function on the ring.
2,6-Bis(3-methylbutyl)-4-(3-methylbutanoyl)resorcinol (6) has been applied to confirm
the enolization pattern in humulone (see 2.1.1.2.1.).
3.6. 2,4,6-TRIS(3-METHYLBUTYL)RESORCINOL
Further reduction of tetrahydrohumulone leads to the trialkyl derivative
2,4,6-tris(3-methylbutyl)resorcinol (7, Fig. 19) (see 2,1.1.2.1.c). The hydrogenolysis of
the carbonyl group in the 3-methylbutanoyl side chain has been observed for
five-membered ring hop derivatives (17) (see 6.3. and 8.4.1.3.), but the present case is
unique for six-membered ring hop series.
3.7. REFERENCES TO CHAPTER 3.
1. F. Alderweireldt, De Preparatieve Tegenstroomverdeling. In Dutch. University
Gent Lab. Org. Chem., Ph.D. Thesis, 1961.
2. D. De Keukeleire, University Gent, Lab. Org. Chem., Ph.D. Thesis, 1971.
3. F. Borremans, M. De Potter, D. De Keukeleire, Org. Magn. Reson., 7 (1975) 415.
4. M. Verzele, M. Anteunis, Bull. Soc. Chim. Beiges, 68 (1959) 315.
5. D.H.R. Laws, The Brewer, 61 (1975) 12.
6. M. Anteunis, M. Verzele, Bull. Soc. Chim. Beiges, 68 (1959) 705.
7. D. De Keukeleire, M. Verzele, Tetrahedron, 26 (1970) 385.
8. W. Wollmer, Ber., 58 (1925) 672.
9. J.F. Carson, J. Am. Chem. Soc.,73 (1951) 1950.
10. W. Riedl, J. Nickl, Ber., 91 (1958) 1838.
11. M. Anteunis, M. Verzele, Bull. Soc. Chim. Beiges, 68 (1959) 476.
12. B.R. Trapnell, Quart. Rev., 8 (1956) 419.
13. W. Riedl, E. Leucht, Ber., 91 (1958) 2794.
14. N.J. Turro, Modern Molecular Photochemistry. Benjamin/Cummings, Menlo Park,
California, 1978.
15. CM. Fernandez, J. Chem. Soc. Chem. Commun., 1212(1967).
16. M. De Potter, University Gent, Lab. Org. Chem., Ph.D. Thesis, 1973.
17. P.M. Brown, G.A. Howard, A.R. Tatchell, J. Chem. Soc, 545 (1959).
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CHAPTER 6
REDUCED DERIVATIVES OF THE ISOHUMULONES
General remarks on the reduction of the hop bitter acids, are given in the first
section of Chapter 3.
6.1. DIHYDRO-ISOHUMULONES.
Hydrogenation of trans isohumulone with Adams' catalyst in aqueous disodium
carbonate affords a resinous oil. Trans dihydro-isohumulone (1), (molecular formula
C21H32°5) mav be either compound 85 or compound 86, depending on the position
of the remaining double bond (Fig. 47). The distinction between 85 and 86 was made
via hydrolysis to humulinic acid, which proved that the double bond in the
4-methyl-3-pentenoyl side chain had been hydrogenated preferentially. Thus trans
dihydro-isohumulone, obtained as described above, has been identified as
2-(3-methylbutanoyl)-5-(3-methyl-2-butenyl)-3,4-dihydroxy-4-(4-methylpentanoyl)-2-
cyclopentenone (85).
6.2. TETRAHYDRO-ISOHUMULONES.
Hydrogenation of the mixture of cis and trans isohumulones yields
tetrahydro-isohumulones, which could not be characterized unequivocally (1,2). Two
pathways are possible for the preparation of cis and trans 2-(3-methylbutanoyl)-
5-(3-methylbutyl)-3,4-dihydroxy-4-(4-methylpentanoyl)-2-cyclopentenones or the
tetrahydro-isohumulones (87 and 88 respectively, Fig. 47). The method via
isomerization of tetrahydrohumulone should necessarily be followed by separation of
the tetrahydro-isohumulones. On the other hand, hydrogenation of the individual
isohumulones using palladium on carbon as catalyst affords the respective isomeric
tetrahydro-derivatives (molecular formula C21H34O5) (3,4). The spectrometric data
resemble very much those of the corresponding isohumulones, except for the absence
of the 1H-NMR signals of the sp2-methine protons and of the methyl groups on the
double bonds. All absorptions occur in a small spectral region between 5 1 and 5 3.
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0 0
OH
89
Fig. 47. Structural formulae of the two dihydro-isohumulones (85,86), cis and trans
tetrahydro-isohumulones (87,88), cis and trans tetrahydro-isocohumulones (90,91),
4-deoxytetrahydrohumulone (89) and 4-deoxytetrahydrocohumulone (92).
An independent structural proof for the tetrahydro-isohumulones was provided by
the synthesis from phloroglucinol via 4-deoxytetrahydrohumulone. The same reaction
sequence has been pursued as described for the synthesis of 4-deoxyhumulone (see
2.4.1.). The tetrahydro-isohumulones have been isolated by chromatography (4).
The cis and trans tetrahydro-isocohumulones (5) or 5-(3-methylbutyl)-
3,4-dihydroxy-4-(4-methylpentanoyl)-2-(2-methylpropanoyl)-2-cyclopentenones (90
and 91, respectively, Fig. 47) were obtained by hydrogenolysis of colupulone (see